I hope that you do realize that 3D applications such as Google SketchUp has the bad habit of almost triplicating the number of polys in a simple mesh. If you use this for large structures, such as bridges, furniture and static objects, without doubt you will have a lot of performance and stability issues that you will need to deal with. Another thing that I have witnessed from one of your videos regarding your previous mod project for Oblivion, is:

- NavMesh creation knowledge/ or lack of time to do it. (Like the bar counter where the NPC would hit it - this is caused by a badly made or non-existant NavMesh path)
- Too much greyness, a city isn't grey at all, you should learn on how to properly use those textures.
- Use vegetation to make the place more "alive"
- Learn how to use lighting sources to add even more visual impact for the mod
- Avoid using Google SKetchUp

I'm lacking time currently, but expect an edit of this post soon.
(Friendly criticism)